1. Field of the Invention
The present invention relates generally to an apparatus for evaluating characteristics of an LSI and a method of evaluating characteristics of an LSI using the apparatus, and more particularly to an apparatus and a method for obtaining a contact between a probe and a signal take-out electrode of an LSI.
2. Description of the Related Art
In general, an LSI includes, as part of wiring, a pad (electrode) connected electrically to an external circuit, etc. The characteristics of the LSI can be evaluated by applying a signal to the LSI, with a probe made of tungsten, etc. put in contact with the pad of the LSI.
FIG. 1 shows the state in which the probe is put in pressure contact with the pad of the LSI.
A wiring material of the LSI is an alloy consisting mainly of aluminum. The surface of a pad 10-1 is covered with an aluminum oxide film 10-2. The oxide film 10-2 prevents electric conduction between the pad 10-1 and a probe 10-3. In order to evaluate the characteristics of the LSI exactly, the oxide film 10-2 must be removed.
There are two conventional methods (a) and (b) of removing the oxide film 10-2 and evaluating the characteristics of the LSI:
(a) A method of mechanically removing an oxide film on the surface of a pad.
As is shown in FIG. 2, a tip portion of the probe 10-3 is put in pressure contact with the surface of the pad 10-1 at an angle less than 90.degree. C. In this case, the tip portion moves over the surface of the pad 10-1 under pressure produced by elasticity of the probe 10-3. As a result, the oxide film 10-2 adhered to the surface of the pad 10-1 is raked off by the moving probe 10-3.
The method (a) makes use of elasticity of the probe 10-3. Thus, if an excessive load is applied to the probe 10-3, the probe 10-3 slides and damages a protective insulating film 10-4 provided around the pad 10-1. In a worst case, the probe 10-3 destroys the pad 10-1 itself. Since the probe 10-3 is situated at an angle to the surface of the pad 10-1, it is not possible to arrange probes at high density. In the case of an LSI having a plurality of pads arranged at a narrow pitch, it is difficult to evaluate the characteristics of the LSI.
(b) A method of chemically attaining contact between a probe and a pad.
This method adopts a mercury contact system employed widely in relays, etc. The electric conduction between the probe and the pad is attained through molten mercury (Hg). According to the method (b), mercury constitutes an amalgam with material of the pad (metal) and accordingly an oxide film on the pad can be removed. In addition, abnormal discharge occurring when the probe comes out of contact with pad can be prevented, and electric contact between the probe and the pad can be stabilized.
Mercury, however, is harmful, as is well known. In addition, mercury has a high vapor pressure and tends to evaporate easily. Thus, if the method (b) is carried out, the characteristics of the LSI must be evaluated within a completely sealed space.